1. Field of the Invention
The present invention relates to a coating type magnetic recording medium in which a magnetic layer is formed on a nonmagnetic support.
2. Description of the Related Art
A magnetic layer of a coating type magnetic recording medium such as a magnetic tape or the like generally contains a magnetic powder, a binder and a lubricant. As such a binder, a vinyl polymer (vinyl chloride, vinyl acetate, and the like) or vinyl copolymer, a cellulose derivative, a polyurethane resin, and the like are used in an appropriate combination according to the purpose of use.
However, a vinyl chloride-vinyl acetate copolymer, which is a typical vinyl copolymer, has high elasticity and good crosslinking properties, and can thus contribute to improvement in durability and running performance of a magnetic recording medium, but it has the fault that dispersibility of the magnetic powder is insufficient.
For nitrocellulose which is a typical cellulose derivative, from the viewpoint of its molecular structure, it is impossible to adsorb (knead) nitrocellulose in a state where a high concentration of magnetic powder required for high dispersion is contained. Therefore, nitrocellulose has good durability, but has the problem of limiting conditions of use. For acetyl cellulose, the use thereof is limited from the viewpoints of dispersibility of the magnetic powder and compatibility with other resins.
Although a polyurethane resin is excellent in dispersibility of the magnetic powder and adhesion to the nonmagnetic support, the production conditions are limited due to an increase in viscosity during preparation of a magnetic coating, thereby causing problems with running performance and durability of a magnetic recording medium.
For the magnetic recording medium, the magnetic powder is generally improved (fine powder and high anti-magnetic properties) for achieving high density and high performance, with corresponding demand for a binder exhibiting high dispersibility, high durability and high running performance.
Therefore, an attempt is made to use a combination of a polyester polyurethane resin and a vinyl chloride-vinyl acetate copolymer as the binder. In this case, the advantages of the respective resins are effectively used for improving adhesion between the nonmagnetic support and the magnetic layer, the dispersibility of the magnetic powder, the durability and running performance of the magnetic medium, etc. to some extent. However, there are problems in that the improvements cannot be said to be sufficient, and thermal disposal is difficult because the binder contains chlorine.
In order to solve this problem, an attempt has recently been made to use a combination of a polyester polyurethane resin and a phenoxy resin as the binder.
On the other hand, as the lubricant added to the magnetic layer of the coating type magnetic recording medium, conventionally, a liquid lubricant such as a fatty acid lubricant, a higher alcohol lubricant, an amide lubricant, a fatty acid ester (salt) lubricant, a silicone oil lubricant, a fluorine lubricant, or the like; a solid lubricant such as carbon black or the like can be used. Any one of these lubricants can be appropriately selected according to the purpose of use of the magnetic recording medium.
However, when a combination of a polyester polyurethane resin and a phenoxy resin is used as the binder, although the problems with a combination of a polyester polyurethane resin and a vinyl chloride-vinyl acetate copolymer can be solved to some extent, there is demand for further improving the dispersion stability of a magnetic coating and the running reliability and durability of a magnetic recording medium using the binder.
In regard to the lubricant, a fatty acid lubricant has the large effect of decreasing friction due to the addition thereof, but it has problems of high interaction with the magnetic powder and other raw materials due to the degree of polarity of the functional groups, and the difficult addition method. Also, in a frictional state, the lubricant is easily exhausted due to contact (solid) friction, thereby causing the problem of decreasing the amount of the lubricant present on the surface at high temperature and high humidity. Therefore, the lubricant itself has insufficient stability.
A fatty acid lubricant exhibits low interaction with the magnetic powder and good stability, but has the problem of the low effect of decreasing friction for the adding amount. There is also a problem in that the friction coefficient at high temperature is easily increased. Although this problem can be solved to some extent by increasing the molecular weight of a fatty acid ester or changing the stereostruction thereof, the new problem of deteriorating compatibility with the magnetic coating occurs.
For a higher alcohol lubricant, the friction decreasing effect deteriorates at low temperature. For a fatty acid salt lubricant, the possibility of reaction with other components in the magnetic coating with the passage of time cannot be denied. For a fluorine lubricant, it is difficult to stabilize dispersion in the magnetic coating, thereby causing the problem of complicating the production process.
Therefore, in the present state, an attempt is made to use a combination of a plurality of different lubricants at various mixing ratios in consideration of the interaction with the binder and the magnetic powder. However, the correlations between factors such as selection of the lubricant, determination of the mixing ratio, the method of adding the lubricant, etc. and the amount of the lubricant present on the surface of the magnetic layer are not sufficiently studied.